Masers (microwave amplification by stimulated emission of radiation), while spawning the laser, have had little commercial or practical success primarily as a result of the difficult operating conditions which have been required to achieve masing. Atomic and free-electron masers need bulky vacuum chambers and pumping apparatus. Solid-state masers, although being excellent amplifiers with extremely low noise figures, and used occasionally in ultra-stable oscillators, are required to be refrigerated to cryogenic temperatures to function. Most solid-state masers also require a strong magnetic field and magnetic shielding to function.
In the article entitled “Room-Temperature Solid-State Maser” by Mark Oxborrow, Jonathan Breeze and Neil Alford published in Nature on 16th August 2012, henceforth referred to as RTSSM, there is described a structure of a maser which is able to exhibit maser activity at room temperature and without any additional magnetic field being applied to the maser crystal. The prototype maser used a crystal of p-terphenyl doped with pentacene at room temperatures, in air, in the earth's magnetic field, and amplified at around 1.45 GHz. The optical pumping mechanism of that prototype exploited spin-selective molecular intersystem crossing (ISC) into pentacene's triplet ground state. Configured as an oscillator, the measured power output of the prototype maser was around −10 dBm. This is approximately 100 million times greater than that of an atomic hydrogen maser, which oscillates nearby in frequency (˜1.42 GHz). In contrast to the teachings herein, the gain medium of an atomic hydrogen maser is a rarefied, spin-polarized vapour of hydrogen atoms confined within an otherwise evacuated storage bulb. In RTSSM, it was stated that the millikelvin spin temperatures generated by means of the intersystem crossing of photo-excited pentacene could be used to realize an amplifier exhibiting 100 times lower noise that the quietest known room temperature microwave amplifier, though exactly how this could be achieved with the paper's prototype maser assembly was not described.
Notwithstanding these advances, the prototype maser reported in RTSSM was able to sustain masing for a period of 350 microseconds only, thus limiting the effectiveness of that maser assembly to only a few specific applications. It was found that applying either a longer or a more powerful optical pumping input to the pentacene-doped crystal of p-terphenyl would not substantially extend the duration of masing. It was also found that increasing the quality factor of the microwave mode that the prototype maser cavity supported, which would increase the same mode's magnetic Purcell factor, would also not substantially extend the duration of masing. This inevitable self-termination of the maser output was neither reported nor analyzed within RTSSM.
The inventor has since discovered that the cause of the self-termination observed with the maser assembly described in RTSSM in particular and a common and severe impediment to the realization of continuously working masers based on the spin-polarized triplet states of optically excited dye molecules more generally, is bottlenecking in the lower maser level. This bottlenecking is itself caused by this level having a decay lifetime out of the triplet state that is substantially longer than the decay lifetime of the upper maser level out of the same. In the case of the maser assembly described in RTSSM, it has been found that the Z sublevel of the triplet ground state of pentacene dissolved in p-terphenyl, where this sublevel served as the lower level of the maser transition, has a decay lifetime that is substantially longer than either the X or the Y sublevels of the same triplet ground state, where in the case of the prototype reported in RTSSM the X sublevel served as the upper level of the maser transition. This disparity of sublevel decay lifetimes causes an excess of the masing material's dye molecules to build up in the lower maser level, so destroying the population inversion generated by intersystem crossing, so terminating maser action. This self-termination due to bottlenecking in the lower maser level is analogous to the behaviour of the nitrogen laser, which is also intrinsically self-terminating. A rigorous, quantitative analysis of maser action in the prototype assembly considered in RTSSM is complicated by the fact that the rates of spin-lattice relaxation between the triplet ground state's three sub-levels are of a similar order of magnitude to the decay rates from these same sub-levels, leading to subtle effects.
In the case of pentacene-doped p-terphenyl at room temperature, albeit at high magnetic field, the substantial differences in the decay lifetimes of the different triplet sublevels are evident from Table II of Sloop, D. J., Yu, H.-L., Lin, T.-S. & Weissman, S. I. Electron spin echoes of a photo excited triplet: Pentacene in p-terphenyl crystals. J. Chem. Phys. 75, 3746-3757 (1981). Here, the value of k0, which is the rate of decay from the Z sublevel of pentacene's triplet ground state, is more than a factor of 10 smaller than the values of k1 and k−1, which correspond to the decay rate of linear combinations of the X and Y sublevels at zero applied d.c. magnetic field. These same authors observed intriguing “inversions” in their experimental data, where the amplitude of a spin echo as a function of time since the firing of an optical pump pulse from a laser would change its sign from positive to negative (or vice versa) at a time of several tens of microseconds after the laser had fired. These inversions evidenced a complex interplay between spin-lattice relaxation versus decay in the removing of differences in the populations of the triplet ground state's sublevels.
Building upon these observations, there are described below methods whereby the problem of bottlenecking in the lower maser level can be either avoided in the first place or remedied.
The inventor has also identified a certain characteristic in the Jablonski energy-level schemes of certain dye molecules that, if exhibited by a candidate maser material, make the material particularly advantageous for masing. This characteristic is explained below.
As its source of required optical pump light, the maser assembly reported in RTSSM used a pulsed dye laser which, by dint of its bulk and low wall-plug efficiency, further limited the effectiveness of this maser assembly with respect to applications. There is taught herein a different and advantageous pump light source based on the principle of fluorescent concentration, which supplies an efficient means of boosting the luminance of a light source, which no system of geometric optics (such as mirrors or lenses) can do.
A quantitative analysis of the noise performance of maser amplifiers has been provided by Clauss, R. C. & Shell, J. S. in Low-Noise Systems in the Deep Space Network (ed. Reid, M. S., JPL, Caltech, 2008), which itself draws upon fundamental formulae derived from first principles in Siegman, A. E., Microwave Solid-state Masers (McGraw-Hill, 1964). Some re-interpretation of the variables used in Clauss and Shell's analysis, as was set up for the analysis of travelling-wave masers, is needed for it to be applicable to cavity masers such as RTSSM's prototype. Assuming the gain of the maser amplifier is high (which can generally be achieved with a cavity maser by trading off against bandwidth) and that the maser does operate well above threshold, the maser's residual noise temperature is given by (the following expression is based on equation 3.5-6 of the Clauss and Shell's article):
                                          T                          ampl              .                                      resid              .                                ≈                                                    T                0                            η                        +                                                            hf                                      ma                    ⁢                                                                                  ⁢                                          s                      .                                                                      k                            ⁢                              r                                  r                  -                  1                                                                    ,                            (        0        )            where T0 is temperature of the maser's resonator (i.e. the temperature of its ohmically conductive walls and/or lossy dielectric materials within it), η≡Q0/Qm is a dimensionless ratio quantifying the degree to which the gain of the maser's amplifying material exceeds the resonator's losses (maser threshold corresponds to η=1), Q0 is the resonator's intrinsic (unloaded) Q or quality factor, Qm is the maser's magnetic Q (see Seigman), h is Planck's constant, fmas.is the maser's operating centre frequency, k is Boltzmann's constant and r≡Nupper/Nlower is the spin population ratio between the upper and lower maser levels. The above equation (0) becomes more complicated for modest η (see Clauss and Shell) but this added complexity does not affect the basic points concerning trends and scalings made here. According to equation (0), even if the maser operates at room temperature, i.e. if T0≈293 K, the reduced temperature T0/η can still be small if, and only if, the maser is operated far above threshold, i.e. if η>>1.
The previous paragraph and its mathematics can be summed up heuristically in words as follows. For the maser amplification process to be low-noise as a whole, the “quiet”, quantum microwave gain provided by cold stimulated emission is required to overwhelm the “noisy” loss caused by dissipative processes in the room-temperature dielectrics and metal surfaces of the microwave resonator. Provided its right-most term on its right hand side is small, equation (0) above indicates that a maser amplifier's residual noise temperature drops approximately linearly with the multiple by which maser threshold is exceeded, i.e. linearly with η. In certain ultra-low noise applications, a multiple of η≧100 would be advantageous. In the experiment reported in RTSSM, the pulsed dye laser exceeded threshold by a factor η≈7, and only then for a few hundred microseconds as noted above. To exceed threshold continuously by large margins, which, as the above analysis demonstrates, is advantageous with respect to achieving low-noise amplification, compels the informed search and optimization of maser assemblies substantially different to that described in RTSSM.